Dampers, shock absorbers, brakes and clutches are known which use a fluid as the working medium to create damping forces/torques to control motion, shock, and/or vibration. One class of these devices are controllable and the devices employ Electrorheological controllable fluids (ER), Electrophoretic fluids (EP), Magnetorheological fluids (MR), and Hydraulic fluids (Semi-Active Electro-mechanical), etc. Of particular interest are Magnetorheological (MR) fluid devices. MR fluid devices may be of the rotary or linear-acting variety, such as MR dampers, MR brakes and MR clutches. They employ an MR fluid comprised of small soft-magnetic particles disbursed within a liquid carrier. Typical particles include carbonyl iron having various shapes, but which are preferably approximately spherical, and which exhibit mean dimensions of about between about 0.1 to 500 .mu.m, and more preferably between 1 and 100 .mu.m. The carrier fluids include various known hydraulic oils, silicone oils, and the like. These MR fluids exhibit a "thickening" behavior (a rheology change), sometimes referred to as an "apparent viscosity change", upon being exposed to a magnetic field of sufficient strength. The higher the magnetic field strength to which the MR fluid is exposed, the higher the damping force that can be achieved in the particular MR device. Examples of prior art fluids can be found in WO 94/10694, WO 94/10693, and WO 94/10692 the inventions of which are commonly assigned to the assignee of the present invention. Notably, MR fluid devices provide ease of controllability through simple variations in electrical current supplied to the devices and the fluids and devices have demonstrated durability yet unobtained with ER devices (ER fluids exhibit a rheology change upon being exposed to an electric field) and simplicity previously unachievable with controllable semi-active hydraulic devices (which include electromechanical valves).
Descriptions of prior art MR fluid devices can be found in U.S. application Ser. No. 08/304,005 entitled "Magnetorheological Fluid Devices and Process of Controlling Force in Exercise Equipment Utilizing Same", U.S. Ser. No. 08/613,704 entitled "Portable Controllable Fluid Rehabilitation Devices", U.S. Ser. No. 08/610,796 entitled "Controllable Fluid Rehabilitation Device Utilizing a Reservoir of Fluid", U.S. Ser. No. 08/674,371 entitled "Controllable Brake", U.S. Ser. No. 08/674,179 entitled "Controllable Vibration Apparatus" and U.S. Pat. Nos. 5,547,049, 5,492,312, 5,398,917, 5,284,330, and 5,277,281, all of which are commonly assigned to the assignee of the present invention. Notably, these devices provide user-variable/selectable control forces or torques, as the case may be.
Exercise treadmills comprise a deck, a frame, a continuous treadmill belt rotatably entrained and supported about rollers. The rollers are suspended from the frame and a motor and transmission preferably drive the rollers and/or the treadmill belt (the powered type). Generally, the user may control the speed of the treadmill belt to correspond with walking, running, jogging, etc. Notably some treadmills are rotatably engaged by the exertion of the user (the unpowered type). Prior art treadmills have incorporated a passively and flexibly supported deck or various other "passive" means for cushioning the impact of the user with the deck. Certain of these passive deck support systems can be found in U.S. Pat. Nos. 3,689,066, 5,542,892, 4,350,336, 5,382,207, 5,441,468, and 5454,772. Notably, these devices lack any controllability.